Legacy systems exist in various forms. Legacy telephony systems may be limited to only numeric characters and typically only a certain number of digits. In the United States, legacy telephone numbers are ten digits; in Germany, seven; and, in India, thirteen. In contrast, enhanced systems may utilize more digits (e.g., Germany's fifteen digits and India's eight digits). Enhanced systems may also utilize alphanumeric and special characters, such as addresses utilizing a Uniform Resource Locator (URL), Uniform Resource Name (URN), or Uniform Resource Identifier (URI). Compatibility issues between the legacy and enhanced system are commonplace.
Legacy systems are improved to become enhanced systems, such as when countries expand numbering digits (e.g., when India transitioned from a seven-digit public switched telephone network (PSTN) local number to an eight-digit number or Germany's move from the E.164 thirteen digits format to the E.164 fifteen digit format). Often upgraded switches are required to enable parsing of more digits. The switches in a PSTN are commonly upgraded to switches that support the legacy format and, upon an event such as completion of upgrades to a central exchange, the enhanced functionally is then activated.
In the case of enterprise systems, leading digits are dropped from the longer number and are then converted to a shorter number for routing inside the enterprise. While this may allow a call to be presented to an internal component, it does not allow for the longer number to be dialed.
For alphanumeric numbers, systems utilize a mapping table. As a result, an incoming call may be routed internally, but it cannot be dialed out without knowing a corresponding destination number (e.g., an E.164 number).